THE rocks of middle ageof the Mesozoic era(see table of geologic time on page 59) are widely distributed through western America. In general they are softer than the older rocks and have been eroded away near the Grand Canyon. However, they are conspicuous where they have escaped the vigorous erosion of Colorado River and its tributary streams.
The middle age is subdivided into the Triassic, Jurassic, and Cretaceous periods. The first of these will be illustrated by the Petrified Forest National Monument, in Arizona; the second by Zion National Park, in Utah; and the last by Mesa Verde National Park, in Colorado.
The rocks of Triassic age in the high plateaus of western America consist chiefly of red shale and sandstone. They form two groups, the lower or older group containing shells of marine mollusks, and the upper or younger group containing fossil wood and the bones of land animals. These fossils show that the rocks of the lower group were formed when the sea covered parts of western America, and that those of the upper group were formed after this region had been raised above sea level.
The rocks of the lower group, known to geologists as the Moenkopi formation, are so conspicuously red that they have been called the painted rocks. In northern Arizona, near the Petrified Forest, these highly colored rocks are exposed to view as far as the eye can reach. This is a semi-desert region, where there is little grass on the ground under the best circumstances and scarcely a tuft where foraging sheep have passed.
The Painted Desert
Here we find small bunches of sagebrush and greasewood, which shelter jackrabbits and their enemies, the wildcats; and in places bunches of ground cactus, near which may be seen large and brilliantly colored lizards, as well as small, dull-colored ones, which dart here and there with incredible swiftness. Also here may be seen the clumsy little horned toads that are found in so many places on the western plains.
A few larger plants, such as the yucca, commonly called soapweed, which consists of a bristling mass of bayonets, grow here. Occasionally a desert sparrow may be seen, or one of the little owls that live in burrows deserted by prairie dogs. One who is especially fortunate may sometimes see in the distance a predatory coyote.
But the principal features that attract the traveler's attention are the barren surface, the highly colored rocks, and the peculiar forms of erosion which characterize the Painted Desert. There is too little rainfall here to wash the road, but the vagrant winds keep it clear of dust in some places and pile up heaps of sand in others. From this naturally paved highway the traveler views the gorgeous landscape. He has the joy of a wide horizon, the bluest of blue skies, and clear, bracing air. No tree shuts off his view, and he can see no human habitation as far as his eye can reachjust colored rocks and sand and cactus and blue sky.
The Painted Desert presents varied and highly colored landscapes. There are badland forms, rounded domes, oval ridges, and trenchlike valleys of intricate pattern. Where some layers of the rock are harder than the others there are mesas, buttes, towers, and monuments. Leading up to the mesas are steps, with hard rocks forming the treads and soft rocks the risers. There are bands of yellow, gray, drab, lavender, pink, lilac, and brown in numerous shades. There are patches of blue and green and white, and even of black. There is every conceivable combination of colors.
The upper group of Triassic rocks, known to geologists as the Chile formation, is scarcely less brilliant than the lower group, but its crowning glory lies in the petrified wood which it contains, best seen in the well-known Petrified Forest of Arizona.
Some readers will associate this forest with Holbrook, others with Adamana, the station at which they left the train to view the stone trees.
The name Adamana recalls Adam Hanna, one of the pioneers of the West, a rough-and-ready character quite able to take care of himself in a lawless countryand for that matter to take care of some of the lawless members of the country, as his long record in the office of sheriff testifies. He was the first to entertain visitors to the Petrified Forest south of his ranch. An article published in 1889 records his complaint that only a dozen people that year saw the wonders which he stood ready to show them while 3,000 visited Yellowstone National Park. These figures are illuminating when compared with those for 1925, when more than 55,000 people visited the Petrified Forest and 154,000 entered Yellowstone Park.
Stone Trees and Wild Stories
The Petrified Forest is justly rated as one of the world's great wonders. Petrified wood is found at many other places in Arizona and elsewhere, but nowhere else are seen such enormous quantities of beautifully colored and perfectly preserved forms of great forest trees (Plate XXII). Many of the trunks are petrified in agate, chalcedony, and jasper; a few in rocks of the same age elsewhere are preserved in copper and iron. Some are turned partly to coal; in others some of the woody material was replaced by silver and by carnotite, a radium-bearing ore.
Some visitors, on entering the Petrified Forest, are much disappointed when they fail to find the stone trees standing upright. Guides can scarcely be blamed for repeating to credulous visitors the burlesques on stories told by irresponsible natives to tourists who are eager to hear strange tales as well as to see strange sights. A favorite yarn tells of "fossil trees in fossil leaf with fossil birds singing fossil songs in the petrified branches." Some even add that a fossil bird shot in mid-air by a fossil hunter did not fall because "even the force of gravity is petrified here." A source of enjoyment not appreciated by the joking guide is his use of the word, which he pronounces "peat-re-fied."
The petrified wood was first brought to the attention of white men by Lieutenant Whipple in 1853. One of the original transcontinental railway routes, the one now utilized by the Santa Fe system, was surveyed near the forest, and since the railroad was built the forest has become better known.
But although white men were slow in making the wonders of this region generally known, it was familiar to the aborigines for untold generations. Archeologists who have studied the records left by them believe that the region was the scene of four somewhat distinct civilizations, extending from the Pueblo Indians, who still inhabit the Southwest, back to the Aztecs. Something is known of two of these civilizations but very little of the other two. The records are of several kinds, including implements, potsherds, pictographs carved on smooth faces of rock, and ruined buildings.
Some of the prehistoric dwellings were made of petrified wood. Never were buildings, ancient or modern, more truly unique, and never were palaces built of more beautiful material, for these prehistoric homes were made of great brilliant gems of jasper, chalcedony, and agate. The prehistoric men selected their building material from the remains of still older prehistoric trees, for just as the Aztecs belong to a race now dead, these stone trees belong to a vegetable world which has left no living species.
These ruined Indian houses, built of selected pieces of fossil logs, cemented together with clay, gave rise a few years ago to an unfounded story that the dwellings of a prehistoric race had been constructed of trees then living, and that the ruins are "petrified log houses."
Ancient and Modern Uses of Petrified Wood
But the prehistoric use of the petrified wood did not cease with the building of dwellings. From it were manufactured arrow heads and knives and stone hammers. These implements found their way through the narrow channels of Aztec commerce over hundreds of miles. This commerce was checked by the extinction of the race.
The modern traffic in petrified wood, which for a time threatened to destroy the natural aspect of the Petrified Forest, was happily checked in a better way, when the tract of land containing the best of the stone trees was set aside as a national monument. Before the commercialism was checked many carloads of the material were shipped away and cut into ornaments, and machinery had been sent to Adamana for the purpose of crushing the fossil trees into abrasive material for the manufacture of sandpaper. Fortunately the machinery was never operated for this purpose.
To one entering the forest a strange and at first puzzling scene presents itself. The fragments of petrified wood cover the surface of the ground in great profusion. There are myriads of small pieces or chips, and in some places, especially in the hollows, the ground is literally paved with cross sections of the stone logs resembling blocks of stove-wood. Only easily gullible persons will listen to statements that these so-called chips result from the activities of prehistoric men wielding their stone axes. The "chips" are simply fragments of petrified wood broken from the logs on their exposure to the air, chiefly by expansion and contraction under the influence of heat and frost. The sections range in length from several feet down to an inch or two. Some of them are so related to one another that they were obviously derived from the same log, but most of them lie in disorder so great that the forest may appropriately be called ruins. At few places are logs seen in the rocks.
Where did all the petrified wood seen at the surface come from?
This question is answered when we go to the forest, where a natural bridge is formed by an unbroken fossil log. This log is 4 feet in diameter at the base and 18 inches in diameter at the top, and is 111 feet long. It now spans a ravine 30 feet wide and 20 feet deep. At both ends the log is embedded in the rock in which it was originally buried. In technical language this petrified tree is "in place." Others may be found in place in the same rocks near the tops of the mesas.
If this great log were not artificially supported a time would soon come when the walls of the widening gulch would no longer support it and it would fall, probably breaking into large segments. These would lie on the floor of the valley until the stream undermined them again and would then fall or roll to a new and lower position. As this process was repeated again and again the segments would be scattered more and more widely.
In a similar manner were formed the valleys just mentioned and the disordered accumulations of petrified wood. The fragments, some of which now lie in jumbled masses, once formed logs similar to that of the bridge and were embedded in rocks which once extended across the site of the present valleys. These rocks disintegrated and were slowly washed away to form the valleys, in much the same way that the intermittent stream is now washing away the rock under the bridge. The petrified logs have thus been let down, some of them several hundred feet, and the petrified wood now on the ground reprepresents logs that were originally distributed through a considerable thickness of rock.
Concerning the kind of trees and the manner in which their species are determined, Dr. F. H. Knowlton, of the United States National Museum, says:
This process of silicification, and perhaps subsequent crystallization, was such that in some specimens the original form and texture of the wood cells has been greatly distorted or even destroyed entirely, while in others it is preserved so well that each cell, with its most intimate structural details, is retained with astonishing fidelity. Thin sections of this wood [ground until they are about 0.003 of an inch thick] may therefore be studied under the microscope with almost as much satisfaction as thin sections cut from a living tree. Although a complete microscopical examination has not yet been made of all these woods of the region, all from the area south of the railroad that have been examined prove to belong to a single species which has been described by the writer under the Araucarioxylon arizonicum. That is, it is simply the wood of an ancient Araucaria, which, it is well known, does not now live in the northern hemisphere. In this wood the annual or growth rings are not apparent to the naked eye, but under the microscope they are observed to be present, though rather poorly developed and somewhat obscure, the yearly growths being separated by a layer of 2-5 tangentially compressed cells. In radial section the tracheids are observed to be provided with numerous bordered pits, which are disposed in a single contiguous row, or occasionally in two mutually compressed rows. The medullary rays, as seen in this section, are composed of short, thin-walled cells which, in at least some specimens, are provided with small oval pores. In tangential section the rays are found to be in a single vertical series of from 1 to 22 cells.
Several species of trees are present in the fossil forests, as may readily be seen on examining the microscopic sections, but the only other species that has been described comes from the "north" forest, the small forest north of the railroad. It is quite different from the other species, and while undoubtedly araucarian in character, shows so many points of divergence from the living Araucaria that Dr. E. C. Jeffrey, of Harvard University, by whom it was studied, created for it a new genus. It is known as Woodworthia arizonica. It is remarkable in that it was provided with short shoots which persisted in the wood of the trunk throughout the life of the tree and now show as pits or scars on the surface. It probably was the presence of twin scars, one on either side of and a little below the principal scar, thus suggesting the lateral cicatricules of the leaf scar of one of the great Paleozoic lycopods, Sigillaria, that led Mr. John Muir to name this forest the "Sigillaria" forest. This name was shown to be entirely erroneous and inapplicable by David White, who reports that no Sigillaria or Lepidodendron are present in these forests.
The intense interest in these ancient trees, some of which must have been nearly 10 feet in diameter and more than 200 feet high, has led to many a flight of fancy1 and to many an inference which is either wholly erroneous or is so expressed as to give a wrong impression. On the other hand, interest in the petrified trees has led to published accounts which contain valuable information. Probably the questions most often asked are, How did the logs get to their present position, and why did they turn to stone?
How Wood Turns to Stone
The details of the process by which wood is replaced with silica or other mineral substances are not well understood. In some way the woody substance is removed and its place is taken by mineral matter in such a way that the cellular structure and even the most minute features of the wood are preserved. A piece of agatized wood does not seem readily soluble, yet the agate was all deposited from solution. Silica is slightly soluble in water containing soda and potash. Probably the logs, after being deeply buried by sand and mud, in which they were entombed for millions of years, were permeated by alkaline waters bearing silica in solution, and as the wood decayed it was replaced particle by particle with silica. The brilliant hues are caused by small amounts of iron and manganese, which color the silicaa natural process reproduced artificially in making stained glass. On exposure to the air the logs become still more highly colored by the further oxidation of the metallic coloring material. The logs at the surface are more brilliantly colored than those only recently exposed.
The occurrence of the logs in their present position calls for further explanation. They all lie prostrate. Their ends were battered and their sides were bruised. From most of them the bark was stripped before they were buried. Very few have roots or branches attached. In some places the logs lie in a tangled mass resembling a log jam.
A few stumps have been found. Some of these, now turned to stone, seem to have been buried where they stood rooted in the ancient soil. But at least one stump was found inverted, with roots uppermost. It had been overturned and possibly had floated for a long distance before it was buried in the sand.
The rocks that inclose the logs contain the shells of fresh water mollusks and the bones of ancient crocodiles and other animals supposed to have lived in streams and in marshy places. Some of the slabs of rocks bear ripple marks formed either by shallow water when the beds of mud were soft or by wind when the sand was dry. Sun-cracks and rain-prints are also found, which tell as plainly as printed words could tell of the exposure of the soft material to the storms and the sunshine of that ancient time.
These facts and many others have been considered in working out the story of the plateau country at the time the logs of the Petrified Forest were entombed, and the changes that occurred during this long period. The story thus worked out by the geologist may be told briefly as follows.
Changes in Sea and Land
When the Southwest emerged from the sea in which the limestone (Kaibab) seen at the rim of Grand Canyon was formed, the rocks were exposed to erosion for a time but were again covered by sea water, which came from the Pacific Ocean, probably across California, Nevada, and Utah. This invasion by the sea took place long before the mountains of California were born. The rocks formed in this seathe Moenkopi formationcontain beds of salt and gypsum, which were deposited in land-locked bays. In a well drilled at Adamana, salt water was found in this formation.
Apparently the land in northern Arizona was rising during Moenkopi time, and by the end of that epoch the sea water had been expelled. Then followed a long period of exposure to subaerial erosion, represented by a break or unconformity between the Moenkopi formation and the overlying beds, the lowest of which is called the Shinarump conglomerate. This period of erosion lasted so long that a large part of the plateau country was worn down by rain and stream to a nearly level plain.
Then, for some reason not definitely known, the streams that had planed the country to a general level became unable to carry their load of sand and pebbles. The material dropped by the weakened currents choked the channels and diverted the streams to new courses, where the process was repeated. Thus the Shinarump conglomerate was formed.
Just as happens now during times of flood, some of the trees that grew on the uplands were washed into the rivers, floated downstream, and after long buffeting and grinding were deposited on some shoal, where they became waterlogged and were finally covered with mud and sand.
Weapons of the Wolf God
In the Piute mythology the broken trunks of the stone trees are the weapons of Shinau' äv, the great Wolf God. (In some accounts spelled Shinarav.) The accumulated masses, such as those in the Petrified Forest, are said to mark the battlefields of this warrior god. The presence of petrified wood suggested to Major Powell the appropriateness of the name Shinarump (the weapons of Shinau' äv) for the rocks containing them. This name has since been restricted to the conglomerate, and another name, Chinle, has been applied to the overlying rocks. Prof. H. E. Gregory adds that, to the Navajo Indians the petrified logs are yeitsobitsin, the bones of Yeitso, a monster who was destroyed by the Indians' sun god and whose congealed blood forms the lava flows of the region.
As the plain was built up layer on layer, only the finer material could be handled by the streams. Hence the later depositsthe Chinle formationcontain pebbles in relatively few places. But the decrease in the carrying power of the streams, which prevented them from transporting pebbles, did not affect their ability to carry the floating timber. Hence the logs are found from the bottom to the top of this formation.
Triassic Animals and Plants
The process of building up the plain continued until a thickness of more than a thousand feet of material had accumulated. Where did it all come from and where did the trees grow which were washed out upon the plain?
It is supposed that the sea of early Triassic time was an arm of the Pacific Ocean, which washed the base of the ancient highlands, some of which stood approximately where the Rocky Mountains of Colorado now stand. The giant pines now preserved as stone logs may have grown on the western slopes of the ancient Rockies, as their descendants, the giant sequoias, are growing now in California on the western slope of the Sierra Nevada.
The age of reptiles begins with the Triassic period, although recent discoveries show that reptiles probably originated in the preceding period. The Triassic was a time when reptilian life dominated earth, sea, and air. Land reptiles, swimming reptiles, and flying dragons all lived in Triassic time. Although they did not reach the climax of their development until the following age, the principal types of reptiles lived in Triassic time.
Probably the most notable of these reptiles are the dinosaurs, a name meaning terrible saurians. They constituted the reigning dynasty of the Mesozoic era. Some of these ungainly creatures walked upright and had three-toed feet which made tracks like those of a bird. As birds had not yet developed the famous "bird tracks" of this age were probably made by three-toed dinosaurs.
Some of the reptilian horde became swimmers, such as the sea turtles, the ichthyosaurs, and the plesiosaurs; others developed batlike wings and took possession of the air.
The processes of elimination of the old spore-bearing plants of Carboniferous time and the development of the modern seed bearers, which progressed so rapidly in Permian time, were continued in the Triassic. None of the trees which grew so luxuriantly in Coal Measures time have been found in rocks of Triassic age, although the ferns and rushes continued to thrive. Many species of Triassic cycads are known. These plants, now almost extinct, were so abundant in the Triassic and next younger period that the Mesozoic is sometimes called the Age of Cycads.
But the plants that attract most attention, because of their conspicuous fossil remains found in the Petrified Forest, are the giant pines that grew in southwestern America. These pines lived also in the central and eastern parts of the United States, together with other cone-bearing trees, tree ferns, cycads, and gigantic horsetails. From the fossil pines we get some idea of climate, for they possess rings of growth that indicate growing seasons alternating with periods of rest and therefore change of season.
One of the most interesting of the Triassic plants is the Ginkgo, the maidenhair tree. It has a wonderful history, perhaps a more remarkable history than any other tree now living. It has come to us practically unchanged from earliest Mesozoic time. Apparently it reached its widest distribution in the Jurassic period, when it spread over much of the globe. Since Jurassic time it has been gradually diminishing, until at present it is represented by a single living species, native to Japan and China.
In the far East the Ginkgo is regarded as a sacred tree and as such is planted about the temples and sanctuaries. Possibly it has been preserved in this way, for it is not known anywhere at the present time in a truly wild state. It does not seem likely to become extinct as long as civilized men prize its fascinating history and admire its curious foliage. It is a familiar sight on the streets of Washington, D.C., and is cultivated in many parts of the world.
Close of Triassic Time
As the greater part of North America was above water and undergoing erosion during most of the Triassic period, sedimentary records were then being destroyed over most of the continent. Because of this general destruction of evidence, little can be said definitely of the geologic events that brought the Triassic period to a close. At or near the end of the period great crustal disturbances occurred in many parts of the earth, which resulted in the formation of mountains, in a change of level of broad areas of land, and in the shifting of seas. Just when and how these disturbances occurred may be left to the professional geologist, but their influence on western America is of general interest, for it was this new continent that furnished the material for the Jurassic rocks, which make the most remarkable landscapes of the plateau country.
Last Updated: 31-Dec-2009